Alex Salmond found himself in a strange land – under pressure

If it had been decided just on the closing statements, then Tuesday night's STV debate would have been a triumph for the SNP first minister, Alex Salmond, and Alistair Darling's no campaign would have been feeling pretty sick.

But against all predictions, it was Darling who emerged as a solid winner. There was still no positive vision of Scotland's future as part of the UK, but he raised too many uncomfortable questions about what independence would mean – questions that on the evidence so far worried the key voters who have yet to make up their minds. The ICM instant poll for the Guardian showed a clear win for Darling.

The debate may even have been the game changer that everyone was expecting. Certainly it offered – for those in Scotland, and those south of the border who could make their STV player work – a compelling two hours of political debate between the two versions of Scotland's future. The Commonwealth Games – great Scotland – were over, and the first world war commemorations on Monday in Glasgow – great Britain – had gone. From now until 18 September, the referendum will dominate British domestic politics.

As the contenders lined up behind their lecterns, the polls were suggesting that although the yes campaign may have enjoyed a small bounce from the Games, the no campaign retained its comfortable lead.

So Salmond, nimble, aggressive, witty, was odds-on to win the debate and lose the argument. Darling was equally expected to be trampled underfoot without damaging his side's prospects.

Salmond's challenge was to sell confidence; Darling's to undermine it. When the substance can only be a question of speculation, it had to be a battle of style. Darling had to make negativity appeal over the Salmond pitch to romantic possibility.

It always helps to start as the underdog, as Nick Clegg discovered to his advantage four years ago. But almost from the start it seemed Darling had worked out how to translate "yes but", into a persuasive attack. He succeeded unexpectedly well. Salmond ended up looking if not quite skewered then uncomfortable in an unfamiliar way. His supporters' fear that he might look smug must have evaporated early on.

The no campaign has always believed that as the vote approached, Darling's forensic dullness would switch from liability to asset. His assault on Salmond's emotional appeal to the possibilities that would open up for an independent Scotland, beginning with the question of the pound – not George Osborne's pound, not Scotland's pound but the UK's pound – was followed up by goading Salmond over the absence of a plan B.

Salmond supporters in the studio kept cheering, but when it was the audience's turn to ask questions, they were unexpectedly hostile.

After the third break, Salmond changed tack. He came out from behind his lectern and tried to woo the audience. Instead he was challenged to justify snide remarks, and accused of playing the man not the message.

Somehow, the idea of "Project Fear", previously so effective against the Better Together campaign's negativity, looked more like Project Rather a Good Question. Still not exciting, but, for everyone with a vote in six weeks' time, one that's likely to linger in the mind.